1. Field of the Invention
The invention relates to semiconductor fabrication, and more particularly to a method of forming a self-aligned contact.
2. Description of the Related Art
Forming reliable contact structures for semiconductor devices have become more difficult as feature sizes have decreased and device densities on a chip have increased. For example, the aspect ratio (ratio of depth to width) of contact structures increases as device density increases. As a result, it is becoming increasingly difficult to perform contact etching to a required depth without over-etching in a lateral direction.
In order to more reliably fabricate smaller semiconductor device structures at higher density, self-aligned contacts may be used. Self-aligned contacts improve not only the physical characteristics of a contact, but the electrical characteristics as well. Self-aligned contacts use material properties of structures themselves to prevent or reduce the occurrence of some process errors, such as those described above.
One conventional method of forming a self-aligned contact includes first providing a substrate on which there are at least two MOS devices and then forming an insulating layer, such as silicon oxide, on the two MOS devices. Each of the two MOS devices includes a conductive gate and spacers on the sidewalls of the gate. The two MOS devices have a common source/drain region located between the gates of the two MOS devices. The insulating layer is patterned to form a self-aligned contact opening to expose the common source/drain region. A conductive layer is deposited in the self-aligned contact opening to form a self-align-contact.
However, as feature sizes of the MOS devices decrease, a pitch between adjacent MOS devices also decreases to increase device density, such that spacers of the MOS devices exposed by the self-aligned contact opening may be partially removed during formation thereof and the conductive gate may then be exposed. Exposure of the conductive gates after of the contact opening is undesired since shorts will happen between the conductive gate and the self-aligned contact after formation of the conductive layer in the contact opening.